2001 Audi A6 Antitheft alarm

You will need to have the veicle plugged into a diagnostic computer and have new keys coded to the car i'm afraid !

All A6 from m.Y. 2001 are equipped with Immobilizer Generation III.
Generation III has the same functionality as the previous version. However, in addition to the normal data exchange between the immobilizer control module and ignition key, Gen. III includes data exchange between the immobilizer control module and Engine Control Module (ECM).
Data exchange between the Immobilizer control module and ECM takes place over the vehicle CAN-Bus network.
NOTE:For Immobilizer III, the Engine Control Module (ECM) is actively incorporated into the evaluation and monitoring. The control module for immobilizer is integrated into the instrument cluster, i.E. The control module for immobilizer is replaced together with the instrument cluster. See INSTRUMENT CLUSTER, REMOVING AND INSTALLING.
For immobilizer III, the keys are marked with an inscribed "W". The code consists of a fixed code and a variable code portion.

Immobilizer III consists of:
an adapted control module for anti-theft immobilizer (integrated in instrument cluster)
a warning light in instrument cluster
an induction coil at ignition lock
an adapted Engine Control Module (ECM)
and adapted ignition keys with electronics (transponder).
The anti-theft immobilizer is a system for enabling/locking the Engine Control Module (ECM) via the CAN wire.
The transponder code consists of a fixed code and a variable code. This is different for every start procedure and therefore acts as a copy protection for the transponder.
Every immobilizer contains a different computational rule for the variable code that remains the same for its entire service life. During adaptation of the vehicle key, the immobilizer writes this computational rule into the transponder and simultaneously learns the fixed code of the current key.
The fixed code identifies each individual key so that a key that is lost can be blocked from usage. Each time the ignition is switched on, the induction coil for immobilizer reads the transponder's fixed code and then the variable code and determines whether this key is authorized for vehicle start.
The warning lamp lights up briefly (max. 3 seconds) and then goes out when an authorized vehicle key is used.
When an unauthorized vehicle key is used or when a system malfunction occurs, the warning lamp blinks constantly during ignition "on".